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The VCS-255 Exam, officially titled "Administration of Veritas InfoScale Availability 7.3 for UNIX/Linux," is a credential designed to validate a professional's skills in deploying, configuring, and managing Veritas Cluster Server (VCS). This exam is aimed at system administrators, consultants, and technical support personnel who are responsible for ensuring the high availability of critical applications and services. Passing this exam demonstrates a comprehensive understanding of clustering concepts and the ability to effectively use the Veritas InfoScale Availability suite to minimize downtime and protect against service interruptions. It is a benchmark of proficiency in a mission-critical technology.
Achieving this certification proves that a candidate has the requisite knowledge to install and configure Veritas Cluster Server and manage its day-to-day operations. The VCS-255 Exam covers a broad range of topics, from the fundamental architecture of the cluster components to advanced troubleshooting techniques. For organizations, having certified professionals on their team provides confidence that their high availability infrastructure is being managed according to best practices. For individuals, this certification can enhance career prospects by formally recognizing their expertise in a specialized and highly valued area of enterprise IT. It is a crucial step for anyone serious about a career in high availability.
Preparation for the VCS-255 Exam requires not only theoretical knowledge but also significant hands-on experience. The exam questions are often scenario-based, requiring the test-taker to apply their understanding to solve practical problems. This includes knowing the correct command-line syntax, understanding the purpose of various configuration files, and being able to interpret log files to diagnose issues. The journey to passing the VCS-255 Exam involves a deep dive into the inner workings of VCS, from its communication protocols to its resource management framework, ensuring a well-rounded and practical skill set.
Before diving into the specifics of Veritas InfoScale, it is essential to grasp the core concepts of high availability (HA) and clustering, which are central to the VCS-255 Exam. High availability is a design approach that aims to ensure a system or application is operational for an exceptionally high percentage of the time. The goal is to eliminate single points of failure, so that if one component fails, the service can continue to run without significant interruption. This is typically achieved by implementing redundancy in the infrastructure.
Clustering is the primary technology used to achieve high availability for applications. A cluster is a group of two or more independent servers, or nodes, that work together and are presented to the outside world as a single system. These nodes are interconnected and constantly monitor each other's health. If one node fails due to a hardware or software issue, the applications and services it was running are automatically restarted on another healthy node in the cluster. This process is known as failover. A key aspect of this technology is that the failover process is transparent to the end-users, who may experience only a brief interruption.
The Veritas Cluster Server (VCS) is an advanced clustering solution that automates this entire failover process. It monitors not only the health of the servers themselves but also the health of the applications running on them. This application-level monitoring is critical because an application can become unresponsive even if the underlying server is running perfectly. The VCS-255 Exam requires a thorough understanding of these principles, including concepts like fault tolerance, failover policies, and the distinction between planned downtime for maintenance and unplanned downtime due to failures.
A deep understanding of the Veritas InfoScale Availability architecture is a prerequisite for success in the VCS-255 Exam. The architecture is composed of several key daemons and protocols that work in concert to create a robust high availability environment. These components form a layered stack, with each layer providing a specific function. At the lowest level, we have the communication protocols that allow the nodes to talk to each other. Above that, we have the membership services, and at the top, the high availability engine that manages the applications.
The core components that every candidate must know are the Low Latency Transport (LLT), the Group Membership and Atomic Broadcast (GAB), and the High Availability Daemon (HAD). LLT is responsible for the high-speed, low-overhead communication between the nodes in the cluster. GAB uses the transport services of LLT to manage cluster membership, determining which nodes are active and part of the cluster at any given time. HAD, often called the VCS engine, is the high-level daemon that makes all the decisions about starting, stopping, and failing over applications based on the cluster's state and the configured rules.
Another important component is the agent framework. The High Availability Daemon does not directly monitor applications. Instead, it relies on specialized programs called agents. There is an agent for each type of resource that VCS manages, such as a database, a file system, or a virtual IP address. These agents are responsible for the specific actions of bringing a resource online, taking it offline, and monitoring its health. The HAD communicates with these agents to manage the overall state of the application service groups. This modular architecture is a key concept for the VCS-255 Exam.
The foundation of any Veritas cluster is its communication layer, managed by the Low Latency Transport (LLT) protocol. The VCS-255 Exam will undoubtedly test your knowledge of LLT's purpose and configuration. LLT is a high-performance protocol designed specifically for cluster communication. It bypasses the standard TCP/IP stack to provide fast and reliable heartbeating and data exchange between the cluster nodes. This high-speed communication is essential for the cluster to be able to detect node failures quickly and reliably.
LLT is configured using two primary files: /etc/llthosts and /etc/llttab. The llthosts file is a simple text file that maps a unique node ID number to the hostname of each node in the cluster. This file must be identical on all nodes. The llttab file is more complex and defines the network links that LLT will use for communication. It is best practice to configure at least two private, redundant network links for LLT communication. These links should be dedicated to cluster heartbeating and not used for public network traffic to ensure reliability.
These private links are often referred to as the cluster interconnects. If one of the private links fails, LLT will automatically continue to send heartbeats over the remaining link, preventing a false cluster panic. This redundancy is a cornerstone of a well-designed cluster. The VCS-255 Exam requires you to understand how to configure these files, how to bring up the LLT service, and how to use utilities like lltstat to verify the status of the network links and ensure that all nodes are communicating correctly. Proper LLT configuration is the first step to a stable cluster.
Sitting directly on top of the LLT communication layer is the Group Membership and Atomic Broadcast (GAB) service. GAB has two primary responsibilities that are critical to cluster operation and a key topic for the VCS-255 Exam. First, it determines the cluster membership. It uses the heartbeats provided by LLT to decide which nodes are currently healthy and active participants in the cluster. If GAB stops receiving heartbeats from a node, it will declare that node as having left the cluster and will initiate a reconfiguration event.
GAB's second function is to provide a reliable messaging and locking service for the higher-level cluster components. When a message needs to be sent to all nodes in the cluster, such as a command to change the state of a resource, it is sent via GAB's atomic broadcast mechanism. This ensures that the message is delivered to all active nodes in the same order, which is essential for maintaining consistency across the cluster. GAB also provides kernel-level locking to prevent data corruption in scenarios like a "split-brain," where network failures cause the cluster to partition into multiple independent groups.
The main configuration file for GAB is /etc/gabtab. This file is typically configured by the installation scripts and specifies how many nodes must be present for the cluster to start, a process known as seeding. The command gabconfig -a is a vital troubleshooting tool that shows the status of GAB and lists the various "ports" that are registered with it. These ports represent the different clients using GAB's services, such as the VCS engine itself (port h) and the I/O fencing module (port f). Understanding GAB's role in membership and messaging is fundamental for the VCS-255 Exam.
At the top of the Veritas Cluster Server architecture is the High Availability Daemon (HAD), also known as the VCS engine. This is the master daemon that orchestrates all high availability operations. A solid understanding of HAD's role and function is central to the VCS-255 Exam. HAD is responsible for reading the cluster configuration, interpreting the dependencies between resources, and making all the decisions about when to bring applications online, take them offline, or fail them over to another node.
HAD runs on every node in the cluster, and each instance maintains an in-memory copy of the cluster's state. It receives information about the health of resources from the various agents and receives information about the cluster membership from GAB. Using this information, it continuously evaluates the state of the cluster against the desired state defined in the configuration. If there is a discrepancy, such as a failed resource or a node leaving the cluster, HAD will take the necessary corrective action based on the configured policies.
The hatr daemon works alongside HAD as a helper process. It is responsible for logging events and triggering any custom scripts, known as triggers, that the administrator has configured. When HAD makes a decision, such as initiating a failover, this event is logged via hatr. This provides a detailed audit trail of all cluster activities, which is invaluable for troubleshooting. The interaction between HAD, the agents, and GAB forms the core logic of VCS, and mastering these relationships is a key objective for anyone preparing for the VCS-255 Exam.
The logical constructs used to manage applications in Veritas Cluster Server are Service Groups and Resources. These concepts are fundamental, and the VCS-255 Exam will test your ability to use them to model a highly available application. A Resource is the smallest configurable unit that VCS can manage. It represents a component of the application stack, such as a disk group, a volume, a file system mount point, a virtual IP address, or the application process itself. Each resource type has a corresponding agent that knows how to manage it.
Resources are never managed individually; they are always grouped together into a Service Group. A Service Group is a collection of all the resources that are required for a specific application or service to run. For example, a highly available database service group might contain a disk group resource, a volume resource, a mount resource for the database files, a listener resource, and the database application resource itself. The Service Group acts as a single container that can be brought online, taken offline, or failed over as one cohesive unit.
This model simplifies the management of complex, multi-tiered applications. Instead of managing dozens of individual components, the administrator only needs to manage a single service group. The VCS-255 Exam will expect you to be able to look at a description of an application and correctly identify the necessary resources and how they should be organized into a service group. This ability to model an application is a core skill for a VCS administrator. The service group is the unit of failover in VCS; when a critical resource within a group fails, the entire group is failed over to another node.
Once resources are placed into a service group, the next crucial step is to define their dependencies. Resource dependencies are rules that dictate the order in which resources are brought online and taken offline. This is one of the most important concepts to master for the VCS-255 Exam, as incorrect dependencies can prevent an application from starting correctly or can cause issues during shutdown. The dependencies create a hierarchical tree structure within the service group.
The relationship is a simple parent-child model: a resource depends on another resource. For example, you cannot mount a file system until the underlying volume is available. Therefore, the Mount resource would be configured to depend on the Volume resource. Similarly, you cannot start a database application until its file system is mounted. So, the database Application resource would depend on the Mount resource. This creates a dependency chain: the Volume must be online before the Mount, which must be online before the Application.
When a service group is brought online, VCS starts the resources at the bottom of the dependency tree first and works its way up. When the group is taken offline, the reverse happens: VCS stops the resources at the top of the tree first and works its way down. This ensures a graceful and orderly startup and shutdown of the application. The VCS-255 Exam will likely present scenarios where you must analyze a set of resources and determine the correct dependency structure to ensure the application's integrity.
Before attempting to install Veritas InfoScale Availability, a series of pre-installation checks are mandatory to ensure a smooth and successful deployment. These checks are a critical knowledge area for the VCS-255 Exam. First and foremost, you must verify that the operating system and its specific version and patch level are supported by the version of InfoScale you are installing. This information is available in the official Veritas software compatibility list. Installing on an unsupported OS can lead to unpredictable behavior and is a common source of problems.
Next, the network configuration must be properly planned. As discussed, a Veritas cluster requires at least two private network links for the cluster interconnects. These links should be physically independent, connected to different network switches if possible, to avoid a single point of failure. You must ensure that the network interfaces for these links are configured and operational on all nodes before starting the installation. It is also crucial to plan for the public network, including any virtual IP addresses that will be managed by the cluster for client access to the applications.
Finally, storage considerations are vital, especially for clusters that will manage stateful applications with shared data. You must ensure that the shared storage is visible to all nodes that will be part of the cluster. This involves proper SAN zoning or network configuration for shared storage devices. The LUNs (Logical Unit Numbers) presented to each server should have consistent device naming. Neglecting these pre-installation steps is a common pitfall, and the VCS-255 Exam will expect you to know what constitutes a properly prepared environment for a VCS installation.
The installation of Veritas InfoScale products is managed by a unified product installer. A thorough understanding of this installer's operation is required for the VCS-255 Exam. The installer is an interactive script that guides the administrator through the process of selecting products, configuring licensing, and performing the installation and initial configuration of the cluster components. It simplifies what would otherwise be a very complex series of manual steps. The installer can be run in several modes, including a fully interactive mode for guided setups and a non-interactive mode using a response file for automated or large-scale deployments.
The installation process begins with running the installer script from the software distribution media. The script first performs a series of pre-flight checks on the systems to ensure they meet the minimum requirements. It then prompts the administrator to choose which products to install. For a high availability cluster, you would select Veritas InfoScale Availability. The installer will ask for the system names of all the nodes that will form the cluster and will use SSH or RSH to communicate with the remote nodes to perform the installation on all of them simultaneously.
A key part of the installation process is the configuration of the cluster itself. The installer will prompt for critical information such as a unique cluster name, the network interfaces to be used for the LLT private links, and the virtual IP address for the cluster management console, if applicable. Once all the information is gathered, the installer proceeds to install the software packages, create the necessary configuration files like /etc/llthosts and /etc/gabtab, and start the cluster services. Knowing the key inputs required by the installer is an important aspect of preparation for the VCS-255 Exam.
After the InfoScale installer completes, it is imperative to perform a series of verification steps to confirm that the cluster has been set up correctly and is fully operational. This is a crucial skill for any VCS administrator and a likely topic for questions on the VCS-255 Exam. The first step is to verify the status of the low-level communication protocols, LLT and GAB. These services are the foundation of the cluster, and if they are not working correctly, nothing else will.
To check the status of LLT, you can use the lltstat -n command. This command will display the status of the LLT links on the local node and show which other nodes it is communicating with. You should see all other nodes in the cluster listed with a state of "OPEN." To verify GAB, the gabconfig -a command is used. This will show the GAB membership, and you should see all nodes listed in the "Membership" section. It will also show the clients, or ports, that are registered with GAB, including the vital "port h," which represents the VCS engine.
Once LLT and GAB are confirmed to be healthy, the next step is to check the status of the VCS engine itself, the HAD daemon. The hastatus -sum command provides a high-level summary of the cluster's state. On a newly installed cluster with no applications configured yet, this command should show the state of each system as "RUNNING." If you see any other state, it indicates a problem that needs to be investigated by checking the VCS log files. Being proficient with these verification commands is essential for the VCS-255 Exam.
The heart of a Veritas Cluster Server configuration is the main.cf file, located in the /etc/VRTSvcs/conf/config/ directory. This text file contains the definitions of all the objects that make up the cluster, including the systems, service groups, resources, and their dependencies. The VCS-255 Exam requires a deep understanding of the syntax and structure of this file, as it is often necessary to view or edit it directly. While many configuration changes can be made via the command line or a GUI, the main.cf file is the ultimate source of truth for the cluster's configuration.
The file is organized into stanzas, with each stanza defining an object. For example, a system object is defined with system system_name (...), and a resource is defined with resource_type resource_name (...). The attributes for each object are listed within the parentheses as attribute = value pairs. One of the most important concepts is that the configuration must be writable before changes can be made. This is done using the command haconf -makerw, which makes a temporary, writable copy of the main.cf. After making changes, the command haconf -dump -makero is used to save the changes and make the configuration read-only again.
A typical main.cf file begins with an include directive for the types.cf file, which defines all the available resource types. This is followed by the cluster object definition, which sets global cluster attributes. Then, the file will contain a definition for each system (node) in the cluster. Finally, the bulk of the file will consist of the service group definitions, which contain the resource definitions and the requires statements that establish the dependencies between them. The ability to read and interpret a main.cf file is a core competency for the VCS-255 Exam.
Veritas Cluster Server provides a rich set of command-line utilities for managing and monitoring the cluster. Proficiency with these commands is absolutely mandatory for the VCS-255 Exam, as they are the primary interface for day-to-day administration. The commands are generally intuitive and follow a consistent syntax, often in the format of ha<object> -<action>. For example, hagrp is used to manage service groups, and hares is used to manage resources.
The most fundamental commands are those used to control the cluster services themselves. hastart is used to start the VCS services on a node, while hastop is used to stop them. The hastatus command is used for monitoring. hastatus -sum provides a concise summary of the state of all systems and service groups, making it the go-to command for a quick health check of the cluster. It will show whether systems are running, whether service groups are online or offline, and if any components are in a faulted state.
For configuration changes, the primary tool is haconf. As mentioned, haconf -makerw and haconf -dump -makero are used to control the read/write state of the configuration. The hagrp command is used for managing service groups. For instance, hagrp -display myapp_sg will show the details of the service group named myapp_sg. Similarly, hares -display myapp_ip will show the attributes of the resource named myapp_ip. Mastering these commands is not just necessary for the VCS-255 Exam; it is the foundation of effective VCS administration.
One of the most common administrative tasks is to manually start and stop an application service group. This is necessary for planned maintenance or for controlled application startups. The VCS-255 Exam will expect you to know the commands to perform these actions and to understand the sequence of events that VCS follows. The commands used are hagrp -online and hagrp -offline.
To bring a service group online, you use the command hagrp -online <service_group_name> -sys <system_name>. This command tells VCS to start the specified service group on the designated system. When VCS receives this command, the HAD daemon on the target system begins the onlining process. It consults the main.cf file to determine the resource dependency tree for that group. It then starts the resources at the bottom of the tree first, calling the online entry point for the corresponding agent of each resource. It proceeds up the tree until all resources are online, at which point the service group's state is marked as ONLINE.
To take a service group offline, the command is hagrp -offline <service_group_name> -sys <system_name>. This initiates the reverse process. VCS starts with the resources at the top of the dependency tree (e.g., the application itself) and calls their offline agent entry points. It works its way down the tree, ensuring that components are stopped in a graceful order. For example, it will unmount the file system before taking the underlying volume offline. Understanding this dependency-driven process is crucial for the VCS-255 Exam.
A core function of a high availability cluster is its ability to move an application from one node to another. It is important to distinguish between a manual switch and an automatic failover, a topic that is frequently covered in the VCS-255 Exam. A manual switch is a planned administrative action, while a failover is an automatic response to an unplanned failure.
A manual switch is initiated using the command hagrp -switch <service_group_name> -to <target_system_name>. This command is used during planned maintenance, for example, when you need to apply patches to the active node and want to move the application to the standby node beforehand. When this command is executed, VCS performs a graceful shutdown of the service group on the current node (the offline process) and then brings the service group up on the target node (the online process). It is a controlled and orderly migration of the service.
An automatic failover, on the other hand, is triggered by a fault. If a critical resource within a service group fails its monitoring checks, or if an entire node goes down, VCS will automatically take action. It will first attempt to take the faulted service group completely offline on the original node. Then, it will bring the service group online on another available node in the cluster, as defined by the group's SystemList attribute. The key difference is the trigger: a manual command versus an unexpected fault. The VCS-255 Exam will test your understanding of how VCS handles both scenarios.
During maintenance activities on an application or the underlying system, it may be necessary to temporarily prevent a service group from failing over automatically. For this purpose, VCS provides the ability to "freeze" a service group. This is a critical concept for safe system administration and a key topic for the VCS-255 Exam. When a service group is frozen, its automatic failover capability is disabled. VCS will still monitor the resources within the group, but if a fault is detected, it will not take any action.
To freeze a service group, you use the command hagrp -freeze <service_group_name>. This is often done before performing a risky operation, such as restarting a database that is being monitored by VCS. If you were to restart the database without freezing the group first, the VCS agent would detect that the process has gone down and would immediately declare a fault, triggering a potentially unnecessary failover. Freezing the group allows the administrator to perform the maintenance without interference from the cluster.
Once the maintenance is complete and the application is stable again, the service group must be "unfrozen" to re-enable its high availability protection. The command for this is hagrp -unfreeze <service_group_name>. It is extremely important to remember to unfreeze the group, as leaving it in a frozen state means the application is no longer highly available. The VCS-255 Exam may present scenarios where you need to identify the correct procedure for performing maintenance, which always involves freezing and unfreezing the service group.
Veritas Cluster Server comes with a rich set of bundled agents that provide out-of-the-box support for common application components. A comprehensive knowledge of these standard resource types is essential for the VCS-255 Exam. Each resource type corresponds to a specific agent that understands how to manage a particular piece of the IT stack. For example, the Mount agent knows how to mount and unmount file systems and check their status, while the IP agent can plumb and unplumb virtual IP addresses.
Some of the most frequently used resource types that you must be familiar with include DiskGroup and Volume for managing Veritas Volume Manager objects, which are fundamental for shared storage. The NIC resource is used to monitor the state of physical network interface cards. The Application resource is a generic but powerful agent used to manage any custom application or script by executing specified start, stop, and monitor programs. The Process agent can monitor for the presence of a specific process name or pattern.
Each of these resource types has a unique set of attributes that control its behavior. For instance, the Mount resource has attributes like MountPoint, BlockDevice, and FsckOpt. The IP resource has attributes like Address and Device. Part of the preparation for the VCS-255 Exam involves studying the key attributes for the most common resource types. Understanding which agent to use for a given component and how to configure its critical attributes is a core skill for building a reliable cluster configuration.
Not all resources managed by VCS have a simple online/offline state. Some resources are considered "persistent," meaning they are always expected to be active and are only monitored by the cluster. The VCS-255 Exam requires you to understand this distinction. A prime example of a persistent resource is the NIC resource. A physical network interface card is either up or down; VCS does not bring it "online" or "offline." Instead, it continuously monitors its state.
When you configure a NIC resource, you are telling VCS to watch over a specific network interface. If that interface fails (e.g., due to a cable pull or a faulty card), the NIC resource will be marked as faulted. If other resources in a service group depend on this NIC resource (for example, an IP resource is configured on that NIC), its failure can trigger a failover of the entire service group. This allows VCS to react to failures in the underlying network infrastructure.
Configuring a persistent resource is similar to other resources in the main.cf file, but its behavior is different. It does not have online or offline procedures. Its primary function is to serve as a dependency for other resources and to provide a monitoring point. The VCS-255 Exam may test your understanding of which resource types are typically persistent and how their failure impacts the service groups that depend on them. Recognizing the role of persistent resources is key to designing a comprehensive monitoring strategy.
The Application agent is one of the most versatile and important agents in VCS, and mastering its configuration is critical for the VCS-255 Exam. It is a generic agent designed to manage any application or script for which there is no specialized, bundled agent. It provides a framework for integrating custom applications into the high availability cluster by allowing the administrator to specify the commands or scripts needed to start, stop, and monitor the application.
The key attributes for an Application resource are StartProgram, StopProgram, and MonitorProgram. The StartProgram attribute specifies the full path to a script or command that VCS will execute to start the application. Similarly, the StopProgram specifies the command to gracefully shut down the application. These scripts must be written carefully to handle all possible conditions and should return a zero exit code on success and a non-zero exit code on failure.
The MonitorProgram is a script that VCS executes periodically to check the health of the application. This script should perform a deep health check, not just check if a process exists. For example, a good monitor for a database might try to connect to the database and run a simple query. The monitor script's exit code tells VCS the status of the application. An exit code of zero means the application is healthy. A non-zero exit code indicates a fault. The VCS-255 Exam will expect you to know how to use these attributes to safely manage a custom application.
For any stateful application, managing the underlying shared storage is a critical function of the cluster. The VCS-255 Exam places significant emphasis on the integration between Veritas Cluster Server and Veritas Volume Manager (VxVM). This integration is primarily handled by the DiskGroup, Volume, and Mount resource types. These resources work together to ensure that the correct storage is made available on the active node before the application is started.
The process begins with the DiskGroup resource. This resource is responsible for importing a VxVM disk group on the active node and deporting it when the service group is taken offline or failed over. A disk group is a collection of disks that are managed as a single unit by VxVM. The DiskGroup resource ensures that the disk group is only ever imported on one node at a time, which is essential to prevent data corruption.
Once the disk group is imported, any volumes within that disk group can be started. This is often handled implicitly by the disk group import process. The next step is the Mount resource. This resource depends on the DiskGroup resource (and sometimes a specific Volume resource). Its job is to mount the file system, which resides on a volume within the disk group, onto a specified mount point. The dependency chain is crucial: DiskGroup -> Mount -> Application. The VCS-255 Exam will expect you to be able to construct this dependency chain correctly.
Providing a stable network presence for a clustered application is a key requirement. Clients need a single, consistent IP address to connect to the service, regardless of which physical node is currently running it. Veritas Cluster Server manages this using virtual or floating IP addresses. The IP and NIC resource types are the primary tools for this purpose, and their configuration is a core topic for the VCS-255 Exam.
The IP resource is used to manage a virtual IP address. When a service group comes online, the IP agent plumbs this IP address onto a specified physical network interface on the active node. When the service group goes offline, the agent unplumbs the address. This makes the IP address "float" from one node to another along with the application. The IP resource is typically configured to depend on a NIC resource.
The NIC resource, as mentioned earlier, is a persistent resource that monitors the health of the underlying physical network interface. The dependency is configured so that the IP resource requires the NIC resource. This means that if the physical NIC fails, the NIC resource will fault. Because the IP resource depends on it, the fault will propagate up the dependency tree, causing the entire service group to fail over to another node where the network is healthy. This ensures that the application is always accessible through its virtual IP.
Every resource defined in VCS is controlled by a set of attributes. The VCS-255 Exam will test your knowledge of the most important attributes and their impact on the behavior of the cluster. While there are many attributes specific to each resource type, there are several generic attributes that apply to almost all resources and are fundamental to understand.
The Critical attribute is arguably the most important. This is a boolean attribute (it can be set to 1 for true or 0 for false). If a resource is marked as Critical (the default for most), its failure will cause the entire service group to fault and trigger a failover. If a resource is not critical, its failure will simply be logged, but the service group will remain online on the current system. This is used for components that are non-essential to the application's core function.
Other key attributes include Enabled and MonitorInterval. The Enabled attribute allows an administrator to temporarily disable a resource without removing it from the configuration. A disabled resource will not be brought online or monitored. The MonitorInterval attribute specifies how often, in seconds, the agent's monitor program will be executed to check the health of the resource. A related attribute, OfflineMonitorInterval, can be used to periodically check that a resource is truly offline when it is supposed to be. Familiarity with these attributes is essential for the VCS-255 Exam.
While basic commands like hagrp and hares are used for daily operations, a deeper level of cluster administration requires familiarity with more advanced commands. The VCS-255 Exam will expect you to know how to perform administrative tasks that go beyond simple online and offline actions. The haconf command is central to this, as it is the primary tool for modifying the live cluster configuration. For instance, to add a requires dependency between two resources, you would use a command like hares -link res_A res_B. This is the command-line equivalent of editing the main.cf file.
Another important set of commands are hasys and hauser. The hasys command is used to manage system-level attributes and states. For example, hasys -display <system_name> shows the configuration details for a specific node. The hauser command is used for managing VCS user privileges, allowing for the creation of different roles with varying levels of access to the cluster, such as an administrator or an operator. Understanding how to create and manage users is part of securing the cluster environment.
A critical concept when making configuration changes is the read-only and read-write mode of the configuration. Before any modification can be made with commands like hares -modify or hagrp -add, the configuration must be put into read-write mode using haconf -makerw. After the changes are complete, the configuration is saved and returned to read-only mode with haconf -dump -makero. This lock-and-save mechanism prevents accidental or conflicting changes. Proficiency with this workflow is a must for the VCS-255 Exam.
The ability to quickly interpret the state of the cluster is a fundamental skill for any VCS administrator. The primary command for this is hastatus -sum, and the VCS-255 Exam will require you to understand its output thoroughly. This command provides a high-level summary, but it is packed with critical information. It shows the state of each system in the cluster, the state of each service group, and where each online service group is currently running.
The system states you will encounter include RUNNING, which is the normal state, and FAULTED, which indicates a problem with the VCS services on that node. A service group can have several states. ONLINE means the group is running successfully on a node. OFFLINE means the group is intentionally stopped. The most critical state to recognize is FAULTED. A service group enters a FAULTED state when a critical resource within it has failed, and VCS was unable to successfully bring the group online on another node. This requires immediate administrative intervention.
Resources also have their own states, which can be viewed with commands like hares -state <resource_name>. Common resource states are ONLINE, OFFLINE, and FAULTED. An additional state, UNKNOWN, indicates that the agent has not yet been able to determine the resource's status. Being able to read the output of hastatus -sum and drill down to find the specific faulted resource is a key troubleshooting skill that will be tested in the VCS-255 Exam.
When a problem occurs in the cluster, the log files are the most important source of information for diagnosing the root cause. The VCS-255 Exam will expect you to know the location and purpose of the key VCS log files. All VCS logs are located in the /var/VRTSvcs/log directory. The most important log file is the engine log, named engine_A.log. This file contains all the high-level decisions and state changes made by the HAD daemon. It provides a timestamped record of every significant event, such as a resource fault, a failover attempt, or an administrative command being executed.
For each resource type, there is also an agent log, named _A.log. For example, the log for the Mount agent is Mount_A.log. These logs contain detailed information about the actions performed by that specific agent. If a Mount resource fails to come online, the Mount_A.log is the first place to look. It will contain the exact mount command that was executed and any error messages that were returned by the operating system.
When troubleshooting, a common technique is to correlate the timestamps between the engine log and the relevant agent log. You would first look in the engine_A.log to find the high-level event, such as "Resource my_mount has faulted." You would then go to the Mount_A.log at the same timestamp to find the specific error that caused the fault. The ability to navigate and interpret these log files is an indispensable skill for any VCS administrator and a core competency for the VCS-255 Exam.
A high availability cluster should never be put into production without thorough testing of its failover capabilities. The VCS-255 Exam requires an understanding of how to safely simulate failures to verify that the cluster behaves as expected. Performing these tests validates the configuration and builds confidence in the solution. There are several ways to simulate failures, ranging from simple to more complex scenarios.
One of the simplest tests is to manually kill a process that is being monitored by an Application or Process resource. For example, if you have a database running, you can use the kill command to terminate the main database process. Within the configured MonitorInterval, the VCS agent should detect that the process is gone, declare a fault, and initiate a failover of the service group. This tests the application-level monitoring and the failover process.
Another common test is to simulate a network failure by disconnecting one of the public or private network cables from the active node. Disconnecting a public network link should trigger a fault in the NIC resource, leading to a failover. Disconnecting a private interconnect link should not cause a failover if redundant links are configured correctly; VCS should simply log the link failure and continue operating on the remaining link. Forcing a resource to fault can also be done with the command hares -fault <resource_name>, which is a non-destructive way to trigger the failover logic for testing purposes.
Problems at the low-level communication layers, LLT and GAB, can be catastrophic for the cluster, potentially leading to a situation known as "split-brain," where the cluster partitions into multiple segments, each believing it is the only active one. The VCS-255 Exam will test your ability to diagnose these foundational issues. The primary tools for this are the lltstat and gabconfig commands.
If you suspect a network issue, lltstat -l will show the status of the configured LLT links on the local node. lltstat -n will show the status of communication with all other nodes. If a link is down or a node is not visible, it points to a physical network problem, a switch misconfiguration, or a firewall blocking traffic. These issues must be resolved at the network level.
A common and dangerous scenario is when all private links between nodes fail simultaneously, but the nodes themselves are still running. This leads to a split-brain. Each node, unable to hear heartbeats from the others, will believe the other nodes have crashed. GAB will then try to form a new cluster with just itself. If I/O fencing is not configured, both nodes might try to bring the same service group online and import the same shared disk group, leading to severe data corruption. This is why I/O fencing is so critical. The VCS-255 Exam requires you to understand the danger of split-brain and the role of I/O fencing in preventing it.
I/O fencing is a critical technology for preserving data integrity in a shared storage environment, and it is a major topic for the VCS-255 Exam. Its sole purpose is to prevent data corruption in a split-brain scenario. When a cluster partitions due to a network failure, I/O fencing ensures that only one side of the partition can access the shared storage, effectively "fencing off" the other side from the disks.
VCS I/O fencing works by using a coordinator mechanism and SCSI-3 Persistent Reservations (PR). The coordinator can be a set of three dedicated coordinator disks or a CP server. When a node joins the cluster, it registers a key with the coordinator and places a reservation on the shared data disks. If a node loses communication with the rest of the cluster, it will attempt to race to the coordinator to place a lock. The first node to get the lock wins and is allowed to keep its reservations on the data disks.
The losing node, unable to get the lock, knows it is in the minority partition. It will be unable to break the reservations on the data disks held by the winning node. To prevent any possibility of data corruption, the I/O fencing driver on the losing node will then block all write access to the shared disks and cause the node to panic (reboot). This is a safety measure to guarantee data integrity. Understanding this process of registration, reservation, and racing for the coordinator is essential for the VCS-255 Exam.
While a standard Veritas cluster provides high availability within a single data center, many organizations require a disaster recovery (DR) solution to protect against site-wide outages. This is where the Global Cluster Option (GCO) comes in. GCO is an extension to Veritas Cluster Server that allows you to link two or more separate clusters, typically located in different geographical locations, to manage application failover between sites. Understanding the purpose and basic architecture of GCO is an advanced topic that may be covered in the VCS-255 Exam.
In a GCO configuration, each site runs its own local VCS cluster. These clusters are then linked together, and a heartbeat mechanism is established between them over the Wide Area Network (WAN). The application's data is continuously replicated from the primary site to the DR site using a supported replication technology, such as Veritas Volume Replicator (VVR) or storage array-based replication. GCO monitors the health of the primary site's cluster and the status of the data replication.
If a disaster strikes the primary site and the entire local cluster goes down, GCO can detect this failure. The administrator can then initiate a command to fail over the application to the DR site. GCO will orchestrate the process of making the replicated data writable at the DR site and bringing the application's service group online on the cluster at that location. GCO automates the complex steps of a cross-site failover, significantly reducing the Recovery Time Objective (RTO). The VCS-255 Exam will expect a high-level understanding of this DR solution.
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